Bacteria have been defined as minute vegetable cells. Their claim to a vegetable origin is based upon the experimental fact that they have the power of obtaining their nitrogen from ammonia, a property which is not possessed by animals. In form these organisms vary considerably: the most primitive is to be found in the extremely minute round or oval cells which are described as cocci (fig. 74),the variation in size being indicated by the prefix mega for the larger and micro for the smaller cocci, hence the terms megacocci and micrococci. When two circular or oval cells are joined together they are described as diplococci. When a number of cells are united to form a chain, the organism is termed streptococcus. When there is a combination of four cocci the term tetrad, or merismopedia, is applied. When the packet consists of eight divisions a sarcinacoccus is formed. When irregular heaps like bunches of grapes are found, the mass is called staphylococcus, and if irregular masses of cocci are found imbedded in a gelatinous matrix it is called ascococcus. Some micro-organisms present a rod-like character, varying considerably in length; the very short rods with rounded ends are described as bacteria, the longer ones as bacilli, which term is always used when the length of the rod is more than twice its diameter. There are also other forms, distinguished by the terms vibrios and spirilla (fig. 75), and other filamentous forms.

Bacteria and Bacilli. Highly magnified.

Fig. 74. - Bacteria and Bacilli. Highly magnified.

a, Cocci, singly and varying in size. b, Cocci in chains (streptococcus). c. Cocci in masses [staphylococcus). I and e, Cocci in pairs (diplococcus). f, Cocci in groups of four (merismopedia). g, Cocci in packets (sarcina).h, Bac-Serium (Seplicamia haemorrhagicae). i, Bacillus (B.subtilis). k, Spore formation illustrated by Bacillus anthracis.